The Man versus Horse Marathon is an annual race over 21 miles (34 km), where runners compete against riders on horseback through a mix of road, trail and mountainous terrain. The race, which is a shorter distance than an official marathon road race, takes place in the Welsh town of Llanwrtyd Wells every June.
Huh. :)
TL;DR: Idea stems from a 1980s pub discussion somewhere in Wales.
Most times horse wins, even if they get a 15min penalty. First runners win was at 25th race. Cyclists can win too, if allowed to take part in the race.
Looks like they changed the race post-COVID, allowing humans to regularly outcompete horses